1. Field of the Invention
The present invention generally relates to a fluid filter and a process for making it. In particular, the invention is a novel all-metal gas filter with high efficiency and low outgassing characteristics that is useful as a point-of-use filter for semiconductor process gases.
2. Description of the Prior Art
Semiconductor manufacturing is constrained by the limitations of purity. In chemical vapor deposition of the alternating layers of silicon and dopant, a critical aspect of the process involves the absence of any particulate impurities. The presence of a minute particle can destroy an entire silicon wafer representing many dollars of potential end-product. To that end, an entire industry has developed concerned with only one thing--the filtering of the gases that may come into contact with semiconductor product during its formation.
Clean rooms equipped with HEPA (High Efficiency Particulate Air) filters are the first line of defense. Process equipment is located within "clean rooms" that are filled with carefully filtered air. The design of the equipment itself endeavors to minimize particle shedding, outgassing, and contamination from the materials used to transport and deliver high-purity gases such as silane, arsine, hydrochloric acid and phosphine. An important component in these delivery systems is the filter which insures that particulate contamination does not reach the point where the gas is discharged onto the work (point of use). These filters must not only remove any particulate material, but also must not add any gaseous contamination to the high purity gases. In addition, the gas delivery systems must also be as compact as possible to eliminate contamination, both particulate and gaseous, which might arise from either the installation of such systems, or the normal wear associated with usage. This is especially the case with highly corrosive gases such as hydrochloric acid. Therefore the filters must not only remove particulate material and not be a contributor of gaseous impurities, but they must also be as compact as possible and have small internal and filter volumes.
Various filters are used for filtration of such gaseous fluids to insure ultra high levels of purity in terms of particulate contamination. These include: organic membrane filters, ceramic filters, filters formed from porous metal structures and filters formed from metal fibers. Although some of these various filter media are capable of providing particulate contamination control to levels less than one part per million or greater in terms of particulate control, they are characterized by large filter areas. Due to the large flow area required to sustain flow at reasonable pressures and maintain low face velocities to insure particulate retention, gaseous impurities such as moisture, oxygen and especially hydrocarbons are often present in detectable levels (parts per million). This contamination can occur during manufacture of the filter, during installation of the filter when it is exposed to an atmosphere other than a high purity gas, or even as a result of outgassing from the material the filter is packaged in. In addition, large filter volumes require relatively larger housings to contain them. This in turn results in a greater likelihood of contamination due both to installation and usage and the need for larger gas delivery systems to fit the filters.
Present metal filters include stainless steel, nickel, or nickel alloy sintered-powder types such as the Wafergard.RTM. II SF ( Millipore Corporation, Bedford, Mass.), and the Mott GasShield (Mott Metallurgical Corporation, Farmington, Conn.) (See U.S. Pat. No. 5,114,447, Davis) line of filters. Such filters, being all metal, exhibit low outgassing, high efficiency, corrosion and temperature resistance, low porosity and gas throughput, and high structural strength. However, low porosity has continued to be a drawback for typical sintered metal powder filter elements. Porosities for the above filters range from 40 to 44%, limiting the flow-through characteristics of these filters. The low porosities are inherent in the processes used to manufacture sintered metal powder filters. Typically, the powders are compacted into a mold to form a "green form," then sintered to join the metal particles together to impart the necessary strength. The final filter elements (or "membranes") may be cut from a flat sintered sheet of metallic powder, or molded into the final shape in the molding step. The temperatures at which the sintering proceeds are a critical factor in determining the final porosity. Higher temperatures lead to increased strength, but lower porosity; lower temperatures lead to decreased strength and high porosity. Until now, the final porosity was limited to about 45% in the sintered metal powder art.
There exists a need for increased porosity and gas throughput in the metal filter art. Increased porosity would allow for the construction of smaller filters with all the positive aspects of highly porous metal filters, with less outgassing and particulate shedding problems.